Fault tolerance and failover using active copy-cat

ABSTRACT

Fault tolerant operation is disclosed for a primary instance, such as a process, thread, application, processor, etc., using an active copy-cat instance, a.k.a. backup instance, that mirrors operations in the primary instance, but only after those operations have successfully completed in the primary instance. Fault tolerant logic monitors inputs and outputs of the primary instance and gates those inputs to the backup instance once a given input has been processed. The outputs of the backup instance are then compared with the outputs of the primary instance to ensure correct operation. The disclosed embodiments further relate to fault tolerant failover mechanism allowing the backup instance to take over for the primary instance in a fault situation wherein the primary and backup instances are loosely coupled, i.e. they need not be aware that they are operating in a fault tolerant environment. As such, the primary instance need not be specifically designed or programmed to interact with the fault tolerant mechanisms. Instead, the primary instance need only be designed to adhere to specific basic operating guidelines and shut itself down when it cannot do so. By externally controlling the ability of the primary instance to successfully adhere to its operating guidelines, the fault tolerant mechanisms of the disclosed embodiments can recognize error conditions and easily failover from the primary instance to the backup instance.

REFERENCE TO RELATED APPLICATIONS

The following co-pending and commonly assigned U.S. patent applicationhas been filed on the same date as the present application. Thisapplication relates to and further describes other aspects of theembodiments disclosed in the present application and is hereinincorporated by reference.

-   -   U.S. patent application Ser. No. ______, “MATCH SERVER FOR A        FINANCIAL EXCHANGE HAVING FAULT TOLERANT OPERATION”, (Attorney        Ref. No. 4672/584), filed herewith.

BACKGROUND

Fault Tolerance is generally regarded as the ability to mask, or recoverfrom, erroneous conditions in a system once an error has been detected.Fault tolerance is typically required for mission criticalsystems/applications. Mission critical typically refers to anyindispensable operation that cannot tolerate intervention, compromise orshutdown during the performance of its critical function, e.g. anycomputer process that cannot fail during normal business hours.Exemplary mission critical environments include business-essentialprocess control, finance, health, safety and security. Theseenvironments typically monitor, store, support and communicate data thatcannot be lost or corrupted without compromising their core function.

One exemplary environment where fault tolerance is essential is infinancial markets, and in particular, electronic financial exchanges.The systems that implement an electronic exchange receive and matchorders and otherwise consummate trades so as to implement themarketplace and facilitate the exchanges therein. Consistent reliableoperation is critical to ensuring market stability, reliability andacceptance.

Fault-tolerant typically describes a computer system or componentdesigned so that, in the event that a component fails, a backupcomponent or procedure can take its place with substantially little orno loss of service. Fault tolerance may be provided with software, orembedded in hardware, or provided by some combination. For example, in asoftware implementation, the operating system may provide an interfacethat allows a programmer to “checkpoint” critical data at pre-determinedpoints within a transaction. In a hardware implementation, theprogrammer need not be aware of the fault-tolerant capabilities of themachine. For example, at a hardware level, fault tolerance may beachieved by duplexing each hardware component, e.g. disks are mirrored,multiple processors are “lock-stepped” together and their outputs arecompared for correctness, etc. When an anomaly occurs, the faultycomponent is determined and taken out of service, but the machinecontinues to function as usual.

The level of fault tolerance that is required is typically defined bythe needs of the system requirements, i.e. specifications that clearlystate acceptable behavior upon error, e.g. do errors need to be detectedand corrected or merely detected, and how quickly must such actions betaken?

One method of providing fault tolerance to a system is to add redundancyto one or more of the critical components of the system. Redundantdescribes computer or network system components, such as fans, hard diskdrives, servers, operating systems, switches, and/or telecommunicationlinks that are installed to back up primary resources in case they fail.Three types of redundancy schemes are commonly used for this purpose:

One-for-N (1:N)—There is one standby component for every N activecomponent.

One-for-one (1:1)—There is a standby component for each activecomponent.

One-plus-one (1+1)—This is similar to the one-for-one scheme except thatin the case of one-plus-one, traffic is transmitted simultaneously onboth active and standby components. (Traffic is generally ignored on thestandby.) An example of one-plus-one redundancy is the 1+1 SONET/SDH APSscheme that avoids loss of data traffic caused by link failure.

When providing redundant operation for processing components, votinglogic may be used to compare the results of the redundant logic andchoose which component is correct. For example, in Triple ModeRedundancy, three redundant components may be provided wherein if theresult of one component fails to match the other two, which match eachother, the ultimate result will be that of the two components thatmatched.

A well-known example of a redundant system is the redundant array ofindependent disks (“RAID”). RAID (originally redundant array ofinexpensive disks) is a way of storing the same data in different places(thus, redundantly) on multiple hard disks. By placing data on multipledisks, I/O (input/output) operations can overlap in a balanced way,improving performance. Since multiple disks increases the mean timebetween failures (MTBF), storing data redundantly also increases faulttolerance. A RAID appears to the operating system to be a single logicalhard disk. RAID employs the technique of disk striping, which involvespartitioning each drive's storage space into units ranging from a sector(512 bytes) up to several megabytes. The stripes of all the disks areinterleaved and addressed in order. In a single-user system where largerecords, such as medical or other scientific images, are stored, thestripes are typically set up to be small (perhaps 512 bytes) so that asingle record spans all disks and can be accessed quickly by reading alldisks at the same time. In a multi-user system, better performancerequires establishing a stripe wide enough to hold the typical ormaximum size record. This allows overlapped disk I/O across drives.

There are at least nine types of RAID plus a non-redundant array(RAID-0):

-   -   RAID-0: This technique has striping but no redundancy of data.        It offers the best performance but no fault-tolerance.    -   RAID-1: This type is also known as disk mirroring and consists        of at least two drives that duplicate the storage of data. There        is no striping. Read performance is improved since either disk        can be read at the same time. Write performance is the same as        for single disk storage. RAID-1 provides the best performance        and the best fault-tolerance in a multi-user system.    -   RAID-2: This type uses striping across disks with some disks        storing error checking and correcting (ECC) information. It has        no advantage over RAID-3.    -   RAID-3: This type uses striping and dedicates one drive to        storing parity information. The embedded error checking (ECC)        information is used to detect errors. Data recovery is        accomplished by calculating the exclusive OR (XOR) of the        information recorded on the other drives. Since an I/O operation        addresses all drives at the same time, RAID-3 cannot overlap        I/O. For this reason, RAID-3 is best for single-user systems        with long record applications.    -   RAID-4: This type uses large stripes, which means you can read        records from any single drive. This allows you to take advantage        of overlapped I/O for read operations. Since all write        operations have to update the parity drive, no I/O overlapping        is possible. RAID-4 offers no advantage over RAID-5.    -   RAID-5: This type includes a rotating parity array, thus        addressing the write limitation in RAID-4. Thus, all read and        write operations can be overlapped. RAID-5 stores parity        information but not redundant data (but parity information can        be used to reconstruct data). RAID-5 requires at least three and        usually five disks for the array. It's best for multi-user        systems in which performance is not critical or which do few        write operations.    -   RAID-6: This type is similar to RAID-5 but includes a second        parity scheme that is distributed across different drives and        thus offers extremely high fault- and drive-failure tolerance.    -   RAID-7: This type includes a real-time embedded operating system        as a controller, caching via a high-speed bus, and other        characteristics of a stand-alone computer.    -   RAID-10: Combining RAID-0 and RAID-1 is often referred to as        RAID-10, which offers higher performance than RAID-1 but at much        higher cost. There are two subtypes: In RAID-0+1, data is        organized as stripes across multiple disks, and then the striped        disk sets are mirrored. In RAID-1+0, the data is mirrored and        the mirrors are striped.    -   RAID-50 (or RAID-5+0): This type consists of a series of RAID-5        groups and striped in RAID-0 fashion to improve RAID-5        performance without reducing data protection.    -   RAID-53 (or RAID-5+3): This type uses striping (in RAID-0 style)        for RAID-3's virtual disk blocks. This offers higher performance        than RAID-3 but at much higher cost.    -   RAID-S (also known as Parity RAID): This is an alternate,        proprietary method for striped parity RAID from EMC Symmetrix        that is no longer in use on current equipment. It appears to be        similar to RAID-5 with some performance enhancements as well as        the enhancements that come from having a high-speed disk cache        on the disk array.

Similar to RAID, RAIN (also called channel bonding, redundant array ofindependent nodes, reliable array of independent nodes, or random arrayof independent nodes) is a cluster of nodes connected in a networktopology with multiple interfaces and redundant storage. RAIN is used toincrease fault tolerance. It is an implementation of RAID across nodesinstead of across disk arrays. RAIN can provide fully automated datarecovery in a local area network (LAN) or wide area network (WAN) evenif multiple nodes fail. A browser-based, centralized, secure managementinterface facilitates monitoring and configuration from a singlelocation. There is no limit to the number of nodes that can exist in aRAIN cluster. New nodes can be added, and maintenance conducted, withoutincurring network downtime. RAIN originated in a research project forcomputing in outer space at the California Institute of Technology(Caltech), the Jet Propulsion Laboratory (JPL), and the Defense AdvancedResearch Projects Agency (DARPA) in the United States. The researcherswere looking at distributed computing models for data storage that couldbe built using off-the-shelf components.

The idea for RAIN came from RAID (redundant array of independent disks)technology. RAID partitions data among a set of hard drives in a singlesystem. RAIN partitions storage space across multiple nodes in anetwork. Partitioning of storage is called disk striping. Severalpatents have been granted for various proprietary versions of RAIN.

In databases and processing systems, especially stateful processingsystems which store or accumulate state as they continue to process ortransact, redundancy presents additional complications of ensuring thatthe redundant component is synchronized with the primary component so asto be ready to take over should the primary component fail.

A Hot Standby (HS) is a mechanism which supports non-disruptive failoverof database server system maintaining system availability, i.e. itsability to provide desired service when required, by a second serversystem ready to take over when the main system is unavailable. In thehot standby replication scheme servers usually have two different roles,the first of which is a primary server and the second a secondary(backup, slave) server. The hot standby configuration provides a way fora secondary database to automatically maintain a mirror image of theprimary database. The secondary database on the secondary server isusually of read-only type and it is logically identical to the primarydatabase on the primary server. In case a failure occurs in the primaryserver, the secondary server can take over and assume the role of a newprimary server.

There are several methods for achieving high availability in computersystems that contain databases. One known way to carry out continuoushot standby is to mirror the entire system, i.e. databases and theapplications that use the database. All operations of the system areperformed on both applications of the system. The applications writeeach transaction to their respective databases so both systems arecompletely synchronized at all times. To ensure that the applicationsand their databases are mutually in synchronization, typically amechanism called application checkpointing is used. After each executedoperation, the application ensures by some means that the otherapplication has executed the same operation. In other words, thesecondary database in association with the secondary applicationprecisely mirrors the primary database and application. The applicationlevel mirroring is a good choice for real-time applications whereeverything, including the application processes need to be faulttolerant.

The primary process actually performs the work and periodicallysynchronizes a backup process with the primary process usingcheckpointing techniques. With prior known checkpointing techniques, theprimary sends messages that contain information about changes in thestate of the primary process to the backup process. Immediately aftereach checkpoint, the primary and backup processes are in the same state.

In other prior known checkpointing methods, distinctions betweenoperations that change state (such as write operations) and operationsthat do not change the state (such as read operations) are not made, andall operations are checkpointed to the backup process. Such a system isshown in U.S. Pat. No. 4,590,554 (Glazer—Parallel Computer Systems)where all inputs to the primary are provided via messages and allmessages sent to the primary are made available to the secondary orbackup, essentially allowing the backup to “listen in on” the primary'smessages. Another such system is described in and U.S. Pat. No.5,363,503 (Gleeson—Unisys Corporation) where checkpointing is providedas described in U.S. Pat. No. 4,590,554.

Other prior art, such as that shown in U.S. Pat. No. 4,228,496(Katzman—Tandem Computers), describe that the primary receives amessage, processes the message, and produces data. The produced data isstored in the primary's data space thereby changing the primary's dataspace. The change in the primary's data space causes a checkpointingoperation of the data space to be made available to the backup. Thus,there is frequent copying of the primary's data space to the backup'sdata space, which uses a significant amount of time and memory fortransferring the state of the primary to the backup. It may also resultin the interruption of service upon failure of the primary. The overheadfor such checkpointing methods can have considerable performancepenalties.

Other prior art examples attempt to update only portions of the state ofthe primary that has changed since the previous update, but use complexmemory and data management schemes. In others as shown in U.S. Pat. No.5,621,885 (Del Vigna—Tandem Computers) the primary and backup, which runon top of a fault tolerant runtime support layer (that is, an interfacebetween the application program and operating system) are resident inmemory and accessible by both the primary and backup CPUs used in thedescribed fault-tolerance model. The primary and backup processesperform the same calculations because they include the same code.

U.S. Pat. No. 6,954,877 discloses a system and method for checkpointinga primary computer process to a backup computer process such that ifthere is a failure of a primary process, the backup process can takeoverwithout interruption. In addition, upgrades to different version ofsoftware or equipment can take place without interruption. A lightweightcheckpointing method is disclosed that allows checkpointing of onlyexternal requests or messages that change the state of the serviceinstance, thereby reducing the overhead and performance penalties.

In particular, a computing system provides a mechanism for checkpointingin a fault-tolerant service. The service is made fault tolerant by usinga process pair; the primary process performs the work officially, whileone or more backup processes provide a logical equivalent that can beused in the event of failure. The primary and backup are allowed to belogically equivalent at any given point in time, but may be internallydifferent physically or in their implementation.

Implementation of checkpointing mechanisms requires lots of work fromthe application programmers as the application checkpointing mechanismis a difficult task to implement. Another method for processing hotstandby replication operations is to create a transaction log of theoperations of a transaction run in the primary server, transfer the logto the secondary server and run serially the transferred transaction logon the secondary server. This log is a record of all data items thathave been inserted, deleted or updated as a result of processing andmanipulation of the data within the transaction. The data needs to bewritten to both databases before it can be committed in either of thedatabases. This ensures that data is safely stored in the secondaryserver before the primary server sends acknowledgement of successfulcommit to the client application. An example of this kind of datamirroring system is described in the U.S. Pat. No. 6,324,654 where “Aprimary mirror daemon on a local computer system monitors the writelogdevice (redundant data storage or memory device) for data updates andfeeds the data over a network in the same order in which it is stored toa receiving remote mirror daemon on a remote computer system, which inturns commits the data updates to a mirror device.” In a situation of afailure recovery these primary and secondary mirror daemons transfer thelog to the secondary node where the log is run just as it was in theprimary node. The replicated operations are run serially in thesecondary node which slows down processing speed and hence reducesoverall performance.

Still another mechanism for achieving database fault tolerance is tohave an application connect to two databases. Whenever the applicationexecutes an application function, it commits the related data changes toboth servers. To ensure that the transaction is committed in bothdatabases, the application typically needs to use so called two-phasecommit protocol to ensure the success of the transaction in bothdatabases. If the transaction fails in either of the databases, it needsto fail also in the other databases. Using two-phase commit protocolneeds to be done in the application which makes the application codemore complex. Moreover, distributed transactions are quite common causeto performance problems as the transaction cannot be completed beforeboth databases acknowledge the transaction commit. In this scenario,recovery from error situations can also be very difficult.

Still another way for processing hot standby replication operations isto copy the transaction rows to the secondary node after they have beencommitted on the primary node. This method is a mere copying procedurewhere transactions are run serially in the secondary node. This methodis known as asynchronous data replication. This method is not alwayssuitable for real-time database mirroring because all transactions ofthe primary database may not yet be executed in the secondary databasewhen the fail-over from primary to secondary happens.

Many database servers are able to execute concurrent transactions inparallel in an efficient manner. For example, the server may executedifferent transactions on different processors of a multi-processorcomputer. This way, the processing power of the database server can bescaled up by adding processors to the computer. Moreover, parallelexecution of transactions avoid blocking effect of serially executedlong-running transactions such as creating an index to a large table. Toensure integrity of the database, some concurrency control method suchas locking or data versioning needs to be used to manage access to datathat is shared between transactions. If two transactions try to havewrite access to the same data item simultaneously and versioningconcurrency control is in use, the server either returns a “concurrencyconflict” error to one of the transactions and the application needs tore-attempt executing the transaction later. If locking concurrencycontrol is in use, the server makes one of the transactions wait untilthe locked resources are released. However, in this scenario it ispossible that a deadlock condition, where two transactions lockresources from each other, occurs and one of the transactions must bekilled to clear the deadlock condition. The application that tried toexecute the killed transaction, must handle the error e.g. byre-attempting execution of the transaction.

These concurrency control methods known in the prior art are suitablefor use in the primary server of the Hot Standby database configurationto manage concurrent online transactions of client applications but theycannot be applied in the secondary server of the system. This is becausethe concurrency conflict errors cannot be allowed in the secondaryserver as there is no way to properly handle these error conditions.Because of the absence of a proper Hot Standby concurrency controlmethod, in the prior art replicated hot standby operations are runsubstantially in a serial form in the secondary node. Because operationscannot be executed in parallel, it is difficult to improve secondaryserver's performance without raising problems in data integrity andtransaction consistency. Essentially, a mechanism is needed that allowstransactions to run parallel but that ensures that transactions are notstarted too early and they are committed before dependent transactionsare started.

U.S. Pat. No. 6,978,396 discloses a mechanism to run transactionoperations originating from a primary server used to replicate data inparallel in a secondary server and relates to running concurrent orparallel operations in a secondary server for redundancy, recovery andpropagated transactions. According to the disclosure, executing paralleloperations in a secondary server improves performance and availabilityand how it maintains transaction order and output congruent with theprimary server where transaction operations are originated. A set ofspecific rules is determined. The specific rules are defined on basis ofa “first timestamp” and “second timestamp” attached to each transactionin the primary server and the rules form a “timestamp criteria”. When atransaction meets this timestamp criteria it can be run in parallel withother transactions met the same criteria in the secondary server inaccordance with the instructions set in the specific rules to maintainthe transaction order and output correct.

As can be seen, implementation of fault tolerance in complex processingsystems requires complex logic to ensure that the redundant componentsare synchronized with the primary component so that the backup componentis ready to take over should the primary component fail.

Accordingly, there is a need for a simplified mechanism for providingfault tolerance which reduces the complexities related to ensuring thatthe redundant component is ready to take over for a filed primarycomponent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a flow chart showing the operation of a fault tolerantsystem of FIG. 5 according to one embodiment.

FIG. 2 depicts a flow chart showing the operation of a database for usewith the fault tolerant system of FIG. 5 according to one embodiment.

FIG. 3 depicts a more detailed flow chart showing the operation of thefault tolerant system of FIG. 5.

FIG. 4 depicts a more detailed flow chart showing the operation of thefault tolerant system of FIG. 5.

FIG. 5 depicts a block diagram of a fault tolerant system according toon embodiment.

FIG. 6 depicts a block diagram of a fault tolerant system according toan alternative embodiment.

FIG. 7 depicts a more detailed block diagram of the fault tolerantsystem of FIG. 6.

FIG. 8 depicts an alternate more detailed block diagram of the faulttolerant system of FIG. 6.

FIG. 9 depicts a more detailed block diagram demonstrating operation ofthe fault tolerant system of FIG. 6.

FIG. 10 depicts exemplary operation of a database for use with the faulttolerant system of FIG. 6.

FIG. 11 depicts exemplary data structures utilized by the fault tolerantsystem of FIG. 6.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

The disclosed embodiments relate to providing fault tolerant operationfor a primary instance, such as a process, thread, application,processor, etc., using an active copy-cat instance, a.k.a. backupinstance, that mirrors operations in the primary instance, but onlyafter those operations have successfully completed in the primaryinstance. The disclosed fault tolerant logic monitors inputs and outputsof the primary instance and gates those inputs to the backup instanceonce a given input has been processed. The outputs of the backupinstance are then compared with the outputs of the primary instance toensure correct operation. The disclosed embodiments further relate tofault tolerant failover mechanism allowing the backup instance to takeover for the primary instance in a fault situation wherein the primaryand backup instances are loosely coupled, i.e. they need not be aware ofeach other or that they are operating in a fault tolerant environment.As such, the primary instance need not be specifically designed orprogrammed to interact with the fault tolerant mechanisms. Instead, theprimary instance need only be designed to adhere to specific basicoperating guidelines and shut itself down when it cannot do so. Byexternally controlling the ability of the primary instance tosuccessfully adhere to its operating guidelines, the fault tolerantmechanisms of the disclosed embodiments can recognize error conditionsand easily failover from the primary instance to the backup instance.

To clarify the use in the pending claims and to hereby provide notice tothe public, the phrases “at least one of <A>, <B>, . . . and <N>” or “atleast one of <A>, <B>, . . . <N>, or combinations thereof” are definedby the Applicant in the broadest sense, superseding any other implieddefinitions herebefore or hereinafter unless expressly asserted by theApplicant to the contrary, to mean one or more elements selected fromthe group comprising A, B, . . . and N, that is to say, any combinationof one or more of the elements A, B, . . . or N including any oneelement alone or in combination with one or more of the other elementswhich may also include, in combination, additional elements not listed.

FIG. 5 depicts a block diagram of a fault tolerant system 500 accordingto one embodiment. The system 500 includes a primary instance 502 and abackup instance 504, the backup instance 504 being a substantialduplicate of the primary instance 502. The primary instance 502 mayinclude a software application executing on a processor, a process, athread, a processor or other hardware or software component(s),consisting of, or executing on, one or more processing elements,servers, or the like. The backup instance 504 may include a separateinstantiation of the primary instance 502 or a duplicate thereof, andmay consist of, or execute on, the same or different one or moreprocessing elements, server(s), logical partitions, or the like. It willbe appreciated that the nature of the primary and backup instances 502,504, as being software, hardware or a combination thereof, isimplementation dependent and that the fault tolerant mechanismsdescribed herein are generally applicable thereto. In one embodiment,the primary instance 502 is an instantiation of a multi-threadedsoftware process executing on a sever, or logical partition thereof,having one or more processors or processing cores. The backup instance504 is separate instantiation of the same multi-threaded softwareprocess executing on a separate logical partition or separate server ofa similar type which may or may not be located in the same geographicarea. In general, the primary instance 502 operates to perform abusiness or other function for which it is programmed. For example, theprimary instance 502 may be a match server for a financial exchangewhich receives trade orders and matches those orders so as to consummatetrades between the trading entities of the exchange. As will bedescribed, the backup instance 504, being a substantial duplicate of theprimary instance 502, essentially performs the same programmed businessor other function. In effect, the primary and backup instances 502, 504are loosely coupled. Loose coupling describes a resilient relationshipbetween two or more computer systems that are exchanging data. Each endof the transaction make their requirements explicit and make fewassumptions about the other end.

The primary instance 502 is coupled with a database 506 for the purposeof storing transaction data related to the function(s) performed by theprimary instance 502. Herein, the phrase “coupled with” is defined tomean directly connected to or indirectly connected through one or moreintermediate components. Such intermediate components may include bothhardware and software based components. As will be described, theprimary instance 502 is programmed to complete a transaction with thedatabase 506 for each input it receives for processing according to itsfunction(s) and before it can generate and transmit an output, such as aresponse or acknowledgement, in response to the input. Should theprimary instance 502 be unable to complete the database transaction, itwill internally fail and shut itself down. In one exemplary embodiment,the primary instance 502 must log data, such as regulatory or auditrelated data, regarding each received input to a table in the database506.

A transaction typically refers to a sequence of information exchange andrelated work (such as database updating) that is treated as a unit forthe purposes of satisfying a request and for ensuring databaseintegrity. For a transaction to be completed and database changes tomade permanent, a transaction has to be completed in its entirety. Atypical transaction is a catalog merchandise order phoned in by acustomer and entered into a computer by a customer representative. Theorder transaction involves checking an inventory database, confirmingthat the item is available, placing the order, and confirming that theorder has been placed and the expected time of shipment. If we view thisas a single transaction, then all of the steps must be completed beforethe transaction is successful and the database is actually changed toreflect the new order. If something happens before the transaction issuccessfully completed, any changes to the database must be kept trackof so that they can be undone.

A program that manages or oversees the sequence of events that are partof a transaction is sometimes called a transaction manager ortransaction monitor. In one embodiment, transactions are supported byStructured Query Language, a standard database user and programminginterface. When a transaction completes successfully, database changesare said to be committed; when a transaction does not complete, changesare rolled back, i.e. partly completed database changes are undone whena database transaction is determined to have failed. In IBM's CustomerInformation Control System product, a transaction is a unit ofapplication data processing that results from a particular type oftransaction request. In CICS, an instance of a particular transactionrequest by a computer operator or user is called a task. A commit is thefinal step in the successful completion of a previously started databasechange as part of handling a transaction in a computing system.

In one embodiment, the database 506 is an “ACID” compliant database.ACID (atomicity, consistency, isolation, and durability) is an acronymand mnemonic device for learning and remembering the four primaryattributes ensured to any transaction by a transaction manager. Theseattributes are:

-   -   Atomicity. In a transaction involving two or more discrete        pieces of information, either all of the pieces are committed or        none are.    -   Consistency. A transaction either creates a new and valid state        of data, or, if any failure occurs, returns all data to its        state before the transaction was started.    -   Isolation. A transaction in process and not yet committed must        remain isolated from any other transaction.    -   Durability. Committed data is saved by the system such that,        even in the event of a failure and system restart, the data is        available in its correct state.        The ACID concept is described in ISO/IEC 10026-1:1992 Section 4.        Each of these attributes can be measured against a benchmark. In        general, however, a transaction manager or monitor is designed        to realize the ACID concept. In a distributed system, one way to        achieve ACID is to use a two-phase commit (“2PC”), which ensures        that all involved sites must commit to transaction completion or        none do, and the transaction is rolled back.

As was discussed above, the primary instance 502 is further operative togenerate one or more responses/outputs and/or acknowledgements for eachinput it receives. As will be described below, the primary instance's502 dependence on completing an external transaction with the database506 and its operation to generate at least one response or output foreach input received confirming the successful processing thereof, isutilized by the disclosed fault tolerant mechanisms described herein todetect faults and failover to the backup instance 504 when necessary. Itwill be appreciated that the disclosed embodiments may utilize anyoperationally dependent external transaction and any indicator whichconfirms the processing of a given input of the primary instance 502 toaccomplish the disclosed functionality.

The system 500 further includes fault tolerant logic 514. The componentsof the fault tolerant logic 514 will be described with reference totheir functionality as shown in FIGS. 1-4 which depict flow chartsshowing the operation of a fault tolerant system of FIG. 5 according toone embodiment. The fault tolerant logic 514 may be implemented inhardware, software or a combination thereof and further may includecomputer program logic, processes, threads or combinations thereof whichinteract with the primary and backup instances 502, 504, as well as thedatabase 506 and implement the functionality described herein. The faulttolerant logic 514 may execute on the same logical partitions, serversor processors as the primary and/or backup instances 502, 504 or on aseparate server or processor and interconnected with the primary andbackup instances 502, 504 via suitable means, such as a network or otherinterconnect.

In particular, the fault tolerant logic 514 includes an input receiver518 coupled between the network 508 and the input to the backup instance504, primary failure detection logic 516 coupled with the outputs of theprimary instance 502, the input receiver 518 and the database 506, andbackup failure detection logic 528 coupled with the network 508 (notshown) and output matching logic 530. The input receiver 518 receivescopies of the input(s) which should have also been received by theprimary instance 502 from the network 508. Of course, if there is acommunication or network failure with the primary instance 502, thereceipt of the input(s) by the fault tolerant logic 514 will detect thefault as will be described. The input receiver 518 buffers the input(s)and gates them to the backup instance 504 as will be described. Theprimary failure detection logic 516 monitors the output(s) of theprimary instance 502 and, as will be described, determines if theprimary instance 502 has failed. If the primary failure detection logic516 determines that the primary instance 502 has failed, the primaryfailure detection logic 516 also acts to shut down the primary instance502 and fail over to the backup instance 504, determining whether thereare unprocessed input(s) that the primary instance 502 failed processand then causing the backup instance 504 to take over normal operationsfrom the primary instance 502. The backup failure detection logic 528monitors the output(s) of both the primary and backup instances 502, 504for a given input(s) and determines whether they match or not. In oneembodiment, a mismatch triggers a fault in the backup instance 504. Inan alternate embodiment, a mismatch triggers a fail over from theprimary instance 502 to the backup instance 504 as described herein. Thebackup failure detection logic 528 also checks the network connectivityof the backup instance 504 and determines a fault in the backup instance504 when the network connectivity of the backup instance 504 has beendetermined to have failed.

In operation of the system 500, input(s) are received by, or at leasttransmitted to, the primary instance 502, such as via a network 508. Theinput(s) may be received from, or transmitted by, other entities alsocoupled with the network 508 and/or they may be generated by the primaryinstance 502 itself. As will be described, the fault tolerant logic 514may also generate input(s) to the primary instance 502 to determinewhether or not the primary instance 502 is operating correctly. Thenetwork 508 may include one or more input busses, public or privatewired or wireless networks, or combinations thereof and may furtherfeature security or authentication protocols as well as error detectionand correction protocols. In one embodiment, the network 508 implementsthe TCP/IP protocol suite. It will be appreciated that any networkprotocol and communications technology may be used with the disclosedembodiments. A copy of the input(s) is also received by an inputreceiver 518 of the fault tolerant logic 514, the receipt of which mayoccur substantially simultaneously with the presumed receipt thereof bythe primary instance 502, or within a acceptable margin thereofdepending upon the implementation. In one embodiment, inputs aremulticast on the network 508 to both the primary instance 502 and thefault tolerant logic 514. Multicast is communication between a singlesender and multiple receivers on a network. It will be appreciated thatmultiple inputs may be multicast to the primary instance 502 and thefault tolerant logic 514 and that, due to the implementation of thenetwork 508, the input(s) may be received by the fault tolerant logic514 in a different order and/or at a different time than they arereceived by the primary instance 502. In one embodiment, the network 508includes an order entry bus of a match server of a trading engine usedby a financial exchange. It is a feature of the disclosed embodimentsthat the order of receipt of the input(s) does not matter. As will bedescribed, the input(s) received by the fault tolerant logic 514 arebuffered by the input receiver 518 and gated to the backup instance 504under the control of the fault tolerant logic 514. In this way, as willbe described in more detail below, the fault tolerant logic 514surrounds the backup instance 504 to ensure synchronization with theprimary instance 502 without requiring that the backup instance 504 beaware of the fault tolerant logic 514 external thereto.

As shown in FIG. 1, under normal operating conditions, as input(s)(block 102) are received by the primary instance 502 over the network508 (block 106), the primary instance 502 processes the input(s)according to its programmed function, e.g. matches trader order inputsto consummate trades in a financial exchange. At the completion of, orduring, processing, the primary instance 502 attempts to transact withthe database 506, e.g. to store input related data, such as audit dataor transactional data related to the receipt and/or processing of theinput(s), shown in more detail in FIG. 2 (blocks 202, 204) which depictsa flow chart showing the operation of a database for use with the faulttolerant system of FIG. 5 according to one embodiment. If thetransaction with the database 506 fails for reasons other than aconstraint violation (not shown in FIG. 2), the primary instance 502 mayretry the transaction until it is successful or until a threshold ofsuccessive failures has occurred. If the database transaction issuccessful (blocks 206, 208 of FIG. 2), the primary instance 502generates and transmits one or more responses/outputs and/oracknowledgements, as dictated by its program function, such as to theoriginator of the input(s). For example, where the primary instance 502is a match server, the input(s) may include trade orders and theresponse(s) thereto may include acknowledgements of the receipt of theorders and confirmation of trade execution. In addition to beingtransmitted to its intended recipient, the output(s) of the primaryinstance 502 are also copied to the matching logic 532 of the faulttolerant logic 514. The matching logic 532 includes a buffer 524 and acomparator 526. The buffer 524 holds the output(s) of the primaryinstance 502 until the corresponding output(s) of the backup instance504 are transmitted by the backup instance for a given input(s). Theoutput(s) of the primary and backup instances 502, 504 for a giveninput(s) are then compared by the comparator 526 and the results thereofare reported to the backup failure detection logic 528. It will beappreciated that the comparator 526 may determine an exact match betweenthe compared outputs or a substantial match there between. Further, thecomparator may only compare a portion of the output(s), a hash value, orchecksum, or combinations thereof.

Should the database 506 transaction fail due to a constraint violation(blocks 206, 210 of FIG. 2), the primary instance 502 will enter afailure state (block 114). As will be described in more detail below,the fault tolerant logic 514 is capable of forcing a constraintviolation to be returned by the database 506 to the primary instance 502so as to force the primary instance 502 into a failure state. This isdone for the purpose of disabling the primary instance 502 so that thebackup instance 504 can take over in fault situations, in effect takingadvantage of the primary instance's own internal fault handlingmechanisms. It will be appreciated that causing a constraint violationin response to a database transaction by the primary instance 502 is oneexample of a mechanism for interrupting or inhibiting the primaryinstance 502 from completing an operationally dependent externaltransaction and that other such mechanisms may be available and aredependent upon the implementation of the primary instance 502. Further,while primary instance 502 could also be externally terminated, such asby killing the process, removing power from the executing server, etc.,forcing the primary instance 502 to self terminate or fail soft, allowsfor a cleaner exit, i.e. resources in use by the primary instance 502,such as allocated memory, registers, stack space, etc., can be returnedto the operating system and any ambiguous states, such as uncommitted orunresolved transactions, can be resolved. Further, by forcing theprimary instance 502 to fail, rather than simply cutting off the primaryinstance from communicating or interacting, the continued consumption ofresources by the primary instance 502, and the subsequent effects onother processes, etc., can be mitigated. In addition, by utilizing thereturn of a constraint violation to the primary instance 502, theprimary instance 502 is guaranteed to be halted at a known point and/orin a known, at least external, state and that the sequenced set ofinputs that have been processed by the primary instance 502 prior tofailure can be known or discovered by the fault tolerant logic 514 evenif the primary instance 502 is unreachable or otherwise in aninconsistent state.

It will be appreciated that the fault tolerant logic 516 does notactually need to shut down the primary instance 502 but simply needs toguarantee that the primary instance 502 will not send any moreoperations to the outside world, thereby conflicting with the backupinstance 504 that is taking over. In particular, in the disclosedembodiments, the fault tolerant logic 516 assumes the worst casescenario wherein the primary instance 502 cannot be contacted, killed orotherwise directly impacted. By blocking the primary instance 502 fromcompleting an operationally dependent external operation, such as adatabase transaction, it is guaranteed that the backup instance 504 cantake over for a primary instance 502 that has totally disconnected fromthe network without having to contact that primary instance or attack itin any way directly. Once the database block-out is completed, it doesnot matter to the backup instance 504 if the primary instance 502remains in an unresponsive state or if the primary instance selfterminates—but when the primary instance 502 discovers the block-out viaa constraint violation, it may as well shut down as it is now a uselessprocess and logging information and orderly shutting down allows thosemonitoring the process to note the failure and take appropriate restartsteps.

As was described above, the input(s), i.e. copies thereof, are alsoreceived by the input receiver 518 of the fault tolerant logic 518(block 104). The input receiver 518 buffers the received input(s), suchas in the order of receipt and gates those input(s) to the backupinstance 504 for processing. In particular, for a given input, e.g. n,n−1, n−y, etc., the input receiver 518 monitors the output(s) of theprimary instance 502 to determine when an output(s) corresponding to asubsequently received input(s), e.g. n+1, +2, +x, is transmitted by theprimary instance 502 (block 118). When this occurs, the input receiver518 sends one or more of the given prior input(s), e.g. (n+x)−y, to thebackup instance 504 for processing (block 120). In this way, the backupinstance 504 is always processing behind, but in step with, the primaryinstance 502. Further, the receipt of an output for a subsequentlyreceived input, in one embodiment, ensures that the input waiting forprocessing by the backup instance 504 is currently processing, or hasalready been successfully processed, by the primary instance 502. Thebackup instance 504 then processes the input(s) in the same manner asthe primary instance 502 (described above) (blocks 122, 124, 126, 128,139, 132). However, in circumstances where the primary instance 502 isoperating normally and no faults have been detected, the backup instance504 is prevented from interacting with the database 506 and insteadinteracts with database mimic logic 530 which mimics and returns asuccessful database transaction result back to the backup instance 504.In an alternative embodiment, the backup instance 504 may be programmedso as not to attempt interaction with the database 506, therebyeliminating the need for the database mimic logic 530. Further, whilethe output(s) of the backup instance 504 are provided to the matchinglogic 532 to determine if they match with the corresponding output(s) ofthe primary instance 502, the backup instance 504 is prevented fromotherwise communicating those output(s) to other entities so as not tointerfere with the normal operation of the primary instance 502. Gatinglogic 520, 522, under control of the fault tolerant logic 514, controlswhether the primary or backup instance 502, 504 is permitted to transmitits output(s) to external entities, such as via the network 512, basedon whether there has been a failure detected, etc.

As long as the primary and backup instances 502, 504 continue to operatenormally, processing input(s) and generating the requisite output(s),the system 500 operates as described.

FIG. 3 depicts a more detailed flow chart showing the operation of thefault tolerant system 500 of FIG. 5, and in particular, the primary andbackup failure detection logic 516, 528 with respect to monitoring for,detection and handling of fault situations. As shown in FIG. 3, theprimary and backup failure detection logic 516, 528 implement multipleprocess/event loops/flows and/or threads which monitor for events andtrigger actions based on those events, or the lack thereof, to implementthe desired fault tolerant activity. It will be appreciated that numberof processes, threads, etc. used to implement the describedfunctionality, their execution being in parallel or serially, the natureof the events being monitored and the actions taken in response to agiven event or the lack thereof, are implementation dependent and, inparticular, depend on what events, conditions, or combinations thereof,are defined as faults and what events, conditions, or combinationsthereof, are defined as normal operating conditions. The fault tolerantlogic 514, and in particular, the primary and backup failure detectionlogic 516, 528, will be described with respect to an exemplary set ofevents and conditions that must be satisfied to find a fault in eitherthe primary or backup instance 502, 504, as well as an exemplary set ofactions to take with respect therewith. It will be appreciated thatthere may be other conditions and actions with respect thereto that maybe implemented within the scope of the disclosed embodiments.

As long as no faults have been detected, as will be described, thesystem 500 remains in a normal operating state with the primary instance502 operating and the backup instance 504 lagging behind in step withthe primary instance 502 (block 302). As shown in FIG. 4, under normaloperating conditions, the backup instance 504 output(s) are suppressedfrom being communicated, while the primary instance 502 output(s) areallowed, by the gating logic 520, 522, to external entities, such asover the network 512. Further, the backup instance's 504 access to thedatabase 506 is blocked and successful completion of the backupinstance's 504 database transactions are mimicked by the database mimiclogic 530, or alternatively, the backup instance 504 is programmed tonot interact with the database 506.

On each given iteration of the process flow, which may be determined bya clock, counter, event or other trigger, a determination is made as towhether or not the primary instance 502 has transmitted an output (block304). If one or more outputs have been received, it is determined, aswas described above, whether the outputs correspond to one or moreinputs received subsequent to other prior received inputs (block 118)such that the prior received inputs should have been processed by theprimary instance 502 and therefore can then be sent to the backupinstance 504 for processing (block 120). Further, the received output(s)are buffered pending receiving the corresponding output(s) from thebackup instance 504 (blocks 324, 306). Once the corresponding output(s)are received from the backup instance 504, they are compared with theoutput(s) of the primary instance 502 (block 136). If they match,processing continues. If they do not match, in one embodiment, a faultin the backup instance 504 is determined and the backup instance 504 isplaced into a fail state (block 138). Alternatively, a mismatch betweenthe corresponding output(s) of the primary and backup instances 502, 504may be determined to reflect a failure of the primary instance 502triggering the fail over mechanisms described herein.

If there has been no output from the primary instance 502, it is nextdetermined if a threshold time out has elapsed or been exceeded since anoutput was last detected from the primary instance. In one embodiment, aglobal time out threshold is defined for use by all of the event/processflows described herein which may then utilize the threshold or multiplesthereof for their comparison operations. Alternatively, separatethresholds may be defined for each particular process. In the presentembodiment, if no output(s) have been received from the primary instance502 for a multiple of the time out threshold, e.g. 10 times thethreshold, (block 308) the fault tolerant logic 514 generates aheartbeat input to the primary instance 502, which as described above,is also copied to the backup instance 504 and triggers the mechanismsherein to force some form of output from the primary instance 502. Thisallows the fault tolerant logic 514 to discover whether the lack ofactivity from the primary instance 502 is due to a failure or islegitimate, e.g. it doesn't have any inputs to process. The heartbeatinput will also be received by the fault tolerant logic 514 just likeany other input that is received and will trigger the other faultdetection processes described herein to determine whether the primaryinstance 502 has actually failed.

Additionally, when it has been determined that no output from theprimary instance 502 has been received, a set of conditions are checked,serially (as shown), in parallel, or some other logical arrangement, toconclude that the primary instance 502 has failed. In alternativeembodiments, the conclusion of failure of the primary instance 502 maybe reached by other logical means and based on other conditions and/orevents. In the exemplary embodiment, the conditions that are checkedinclude determining that there is an input that was supposedly receivedby the primary instance 502, as determined by its receipt by the faulttolerant logic 514, that is waiting for an output to be generated (block312). Again, if there is no input to the primary instance 502, therelegitimately will be no output therefrom. In addition, if there is aninput waiting for an output, the age of the input is determined andcompared with a defined threshold age (block 314). If the input is olderthan the threshold age, then no fault will be determined. This preventsoccasionally dropped inputs, such as inputs received during startup ofthe system 500, which may be expected to occur depending on theimplementation and the load on the primary instance 502, from beingprocessed by the backup instance 504 and from causing a failovercondition. This check may be tailored to accommodate the level oftolerance for faults in the system 500 that are deemed acceptableaccording to the implementation, e.g. the acceptable threshold age maybe adjusted and/or the frequency of such faults may be measured andcompared against an acceptable threshold. In systems 500 which cantolerate absolutely no errors or faults, this check may not beimplemented.

Further, it is determined whether a prior output has been received fromthe primary instance 502 which indicates that the primary instance wasfunctioning in the past and prevents a fault condition from beingdetermined when the system 500 is first started (block 316). Again, insystems 500 which cannot tolerate faults, this check may not beimplemented. If these conditions are met, but only one half of thethreshold time out has elapsed, or some other portion thereof (blocks318, 320), the network connectivity of the fault tolerant logic 514and/or backup instance 504 is checked. In one embodiment, a ping signalis transmitted from the fault tolerant logic 514, or the server it isexecuting on, to a recipient such as the primary instance 502 or theserver it is executing on, e.g. a ping utility/process may be usedtransmit a test communication designed to determine the state of networkconnectivity. While processing continues, a separate process (block 316)awaits a response to the ping which would indicate that networkconnectivity is okay. If no response is received, the backup instance504 is placed into a fail state due to presumed loss of networkconnectivity. If the threshold timeout has been exceeded and all of theother conditions have been met, a failure of the primary instance 502 isdetermined (block 302).

As shown in FIG. 4, if a failure of the primary instance 502 isdetermined (block 402), the primary instance 502 is failed over to thebackup instance 504. In particular, the outputs of the primary instance502 are suppressed or otherwise inhibited by the gating logic 520 frombeing communicated while the outputs of the backup instance 504 arepermitted to be communicated by the gating logic 522. In addition, thebackup instance 504 is permitted to transact with the database 506. Theprimary failure detection logic 516 then stores blocking data into thedatabase 506. The blocking data is configured so as to occupy storagethat the primary instance 502 would attempt to also store into. In oneembodiment wherein the primary instance 502 is multithreaded, theprimary failure detection logic 516 is able to handle blocking out Nnumber of asynchronously writing threads that may not be writing insequence and may be writing continuously (attempting to write a block,jump ahead on failure . . . etc). The blocking data will cause thedatabase 506 to return a constraint violation to the primary instance502 should the primary instance 502 still be active and trying toprocess inputs. As was described, the constraint violation should forcethe primary instance 502 to self-fail. Once the primary instance 502 hasbeen blocked, the fault tolerant logic 514 determines which inputs needto be processed by analyzing the inputs received by the input receiver518 that remain unprocessed and also analyzing the database 506 todetermine the final actions of the primary instance 502 prior to failure(block 410). These inputs are then sent to the backup instance 504 to beprocessed (block 412). Normal processing by the backup instance is thenstarted (block 414).

In one embodiment, if the backup instance 504 fails, it may be preventedfrom taking over for a failed primary instance 502. Further, anotherbackup instance 504 may be started to take over for the failed backupinstance 504. In the case of failure of either the primary or backupinstances 502, 504, alerts may be transmitted to other monitoringprograms or processes or to monitoring staff alerting them to the needfor intervention.

By following behind during normal operations of the primary instance502, the backup instance 504 can take care of any unfinished processingby a failed primary instance 502 without having to worry about stayingin sync with the primary instance 502. In operation, the disclosed faulttolerant logic 514 detects failures when the primary instance 502 stopsoperating completely, continues processing but fails to send the properoutput(s) or when the corresponding output(s) of the primary and backupinstances 502, 504 for a given input(s) fail to match.

In one embodiment, the primary and backup instances 502, 504 areinstances of a match engine for a trading engine 600 of a financialexchange, such as the Falcon Trading Engine 600 utilized by the ChicagoMercantile Exchange, as shown in FIG. 6. The Match Engine 602 andDatabase 604 are run on redundant pairs. The system is designed so thatany single Match Engine 602A, 602B or database server can fail withoutan interruption to trading activity. Falcon Match Engine fault-toleranceis handled at the application level by a custom architecture describedherein.

The Match Engine 602 has been designed to run in a paired primary/backupconfiguration as described above. Each individual Falcon Match Engine602A, 602B is one process with a set of loosely coupled threads. Thefunctionality of a complete match engine process is subdivided amongthese threads. This allows an easy parallelization of work over separatephysical processors and use of lower cost hardware.

FIGS. 7 and 8 depicts more detailed block diagrams of the fault tolerantsystem of FIG. 6. The Match Component 602 is the ultimate authority ofthe Falcon engine. It performs the order accepting and matching and alsocontrols the only in-line, transactional persistence in the Falconengine. The Match Component 602 will accept requests for quotes, inboundorders and order cancellations from the Falcon Gateway. It will sendresponses and trade executions out to both the Market Data Component andthe Falcon Gateway. All configuration and management calls are sentthrough the Match Component 602 via the Gateway using, for example, JavaMessaging Service (“JMS”). The Match Component 602 communicates allmanagement and configuration information to the other components via theFalcon internal message manager.

Each Falcon instance 602A. 602B will go through specific engine statesbefore it is ready to accept new incoming orders. The Match Component602 is the ultimate authority on engine state and controls all othercomponents in each Falcon Match Engine server. Market schedules arestored in an Admin database and communicated to the Match Server (atstart-up and through Trading calendar Updates during run-time). Statechanges are then scheduled in the Match Component and initiated by theMatch Component. After a state change occurs, this information iscommunicated to the rest of the components via an AdminOperationmessage. The Admin server can also change the match servers' currentstate by sending an AdminOperation. All administrative Adminoperationscalls are received by the Gateway Component and sent to the MatchComponent. Once the Match Component has acknowledged the AdminOperation,all other components respond in the same manner.

The main execution path of the match component 602 is a single thread.This thread pulls messages off the internal message bus, processes them,and sends out responses. As a result, only one operation is going on inthe match thread at any given time. Regarding persisting in thedatabase, many threads may be used asynchronously and order is restoredafterwards. As the size of a “block” is equal to # of database threads *database batch size, the primary failure detection logic 516, asdescribed above, is able to handle blocking out N number ofasynchronously writing threads that may not be writing in sequence andmay be writing continuously (attempting to write a block, jump ahead onfailure . . . etc).

For a given order book in the match thread, there is a single allocationalgorithm and a configurable amount of overlays. Incoming orders arepassed in sequence first to each overlay, and then to the allocationalgorithm. Each overlay and allocation algorithm then generatesappropriate fills. Once the order has passed through each of the above,the remainder (if any) is placed on the book and fill messages are sentout. There are two overlays in Falcon 1.5 which control the orderallocation for incoming orders—Lead Market Maker (“LMM”) and TOP (withMin/Max). With LMM, orders from certain users get preferentialallocation, say x % of every incoming order. In return for preferentialallocation, those users agree to quote many markets and provideliquidity. With TOP, orders that turn the market [first order at abetter price] get preferential allocation over other orders that join itat the same price. Min and Max are modifiers that determine what MINsize an order must be and what MAX allocation it can get before it losestop status. A book can be configured with all overlays, some overlays,or no overlays at all.

The match thread does not fire timed events (such as group open/groupclose). Instead, these events are fired by a dedicated thread. Once theevent fires, it is converted into a message and placed on the matchthread's queue like any other message. As a result, they are processedone at a time like any other message.

There are only three reasons to persist data in Falcon: Recovery,Regulatory and Surveillance (by the operations staff). Some of this dataneeds to be transactionally written to disk in-line. This means theMatch Server must delay sending out responses while the transactioncompletes.

Additionally, one or more logs of business specific data and systemspecific data are maintained. These logs are not transactional andreside on the local disk. The business specific Log contains allbusiness information that Falcon generates, while the System Logcontains Falcon technical information which is more suited for systemadministrators.

As described, the Falcon architecture will consist of a primary server602A (instance) and an actively running backup server 602B (instance).The backup 602B will have the ability to replicate the primary's stateexactly, and failover with no apparent interruption of service in areasonably quick amount of time.

The maximum delay it should take for the backup to failover is 7-10seconds. Quicker times may be attainable, but this likely is a decisionbased on finding the optimal setting that prevents false positivefailure detection. The failover time will be a configurable parameter.

The system should never send duplicate messages without marking them aspossible duplicates. The system should limit the number of outgoingmessages that are marked possible duplicate. The system will only sendpossible duplicates when caused by failover—during normal operation nonewill be generated.

Falcon Fault Tolerance is based on a concept called Active/Copycat ashas been described above. In this system, both the Backup 602B andPrimary 602A listen for INPUT messages from the Order Entry bus 606. Inone embodiment, the order entry bus 606 is a logical component and theremay be multiple order entry busses 606. In addition, the Backup 602Blistens for RESPONSE messages from the primary 602A. When a RESPONSE fora given INPUT and a response for a subsequently received input arereceived, the Backup 602B then copies the Primary 602A by processing thesame INPUT and compares the Primary RESPONSE to its own OUTPUT. TheBackup server 602B performs the same actions as the Primary server, withthe exception of publishing OUTPUT messages and writing to the database604. In order for Active/CopyCat to work, the following requirements onthe messaging infrastructure do exist:

-   -   All messages sent by Falcon will be uniquely identified by        sender and a monotonically increasing sequence number (per        sender, Falcon server). This is the Event Sequence Number and is        unique to the Falcon instance.    -   All INPUT messages must be uniquely identified.    -   All OUTPUT messages published by Falcon Primary 602A will have        an indication of what message it was in response to. Thus the        Backup 602B can identify the INPUT message which caused the        response.    -   All OUTPUT messages published by Falcon Primary 602A will have        an indication of what input message was processed PREVIOUS to        the input message that resulted in the current output (For        example, all responses to input message 2 will have message 1 in        the previous field).

In primary mode, no fault tolerance specific classes are used. Theprimary 602A is unaware whether a backup 602B exists or not—there is norequirement that a backup 602B need to be run at all, and the backup isa passive listener on traffic that would be sent in any case. As aresult, the described implementation of fault tolerance has noperformance impact on the primary running instance.

In the backup 602B, the actual CopyCat logic checking is done by FaultTolerant (“FT”) Message Managers. These objects allow the business logiccomponents of the system (Gateway, Market Data, and Match Server) to bemostly ignorant of their primary/backup status. Gateway and Market dataneed to be aware of their status only at startup, so that they caninstantiate the correct FT Message Manager. The Match Server needs to beaware of status to enable/disable writing to the database. Regardless ofstatus, each component sends and receives messages as normal; it is theresponsibility that component's FT Message Manager to sequence inboundmessages and suppress output messages in backup mode.

The FT Message Managers are controlled by FT Policy objects, one forOrder Entry and one for Market Data.

The following parameters are configurable in the present embodiment:

-   -   FaultToleranceFailureTimeout—How long should the backup wait        before failing over in the event that failover conditions exist.    -   FaultToleranceInputMessageExpiration—How old should a message be        before it should not result in failover? These messages will not        be forwarded to the engine in case of failover.    -   FaultToleranceMMReceiveTopic—Topic to receive FT pong on.    -   FaultToleranceMMSendTopic—Topic to send FT ping on. If        null/blank, ping is assumed to be successful    -   FaultToleranceHeartbeatInterval—How long should the backup wait        during periods of inactivity before generating an input and        sending it to the primary to determine if it is still alive.

The order entry fault tolerant policy object is responsible forreordering input messages to ensure that they are processed in the sameorder as the primary 602A. It also does verification that primary outputmatches backup output, and initiates failover if necessary. All sourcesof input for the backup falcon 602B send their messages to the orderentry FT policy. This includes the admin server, other order entrygateways, and internal Falcon timer events. As a result, all inputevents can be reordered to the same sequence processed by the primary.

Message flow during normal operation is as follows:

-   -   1. A message is received from a sender and placed in the input        queue.    -   2. One or more output messages are received from the primary        Falcon instance 602A in response to the input message.    -   3. One output messages for the next input message is received        from the primary Falcon instance 602A.    -   4. Since that message indicates that the primary 602A has        successfully processed the previous input message, the prior        input is forwarded to the backup server 602B for processing.

Failover is initiated when all of the following cases are true:

-   -   No primary 602A responses have been received for the failure        timeout.—This indicates that the primary 602A is not talking to        the backup 602B.    -   There is at least one input which has not been responded to by        the primary 602A. This indicates that there is a message that        requires the primary 602A to respond.    -   That input is no older than a configurable number of seconds.        This ensures that a single output dropped by a working primary        602A in a busy market will not cause failover much later in the        day, as long as the primary 602A is processing messages.    -   That input was received at least one failure timeout after the        backup 602B first heard from the primary. This ensures that a        message received by the backup 602B while the primary is being        started will not cause failover.    -   When the failure timeout has half expired, and all of the above        conditions are true, the backup will initiate a ping, such as to        the Admin Server and/or other order entry gateways, to ensure        its network connectivity. Once the entire failure timeout        expires, if that ping was successful, the backup 602B will        initiate failover. If the ping was unsuccessful, the backup will        enter a state where it is unable to take over for the primary        602A going forward.

The backup 602B compares output it produces with responses received bythe primary 602A. If the comparison fails, the backup 602B will enter astate where it is unable to take over for the primary 602A goingforward. All FIX message fields except for timestamp and checksum fieldsare hashed by both the primary 602A and backup 602B, and those hashesare compared. If there is any difference in the messages the backup 602Bwill note it and not attempt to take over for the primary 602A goingforward. All important events are logged at an appropriate log level.

In order to detect failures during periods of low market activity, ifthe backup 602B has not heard from the primary 602A for a configurableperiod, such as 10* the failure timeout, it will generate an inputmessage. The input message is sent to both the primary 602A and backup602B in the same manner as any input from a user. The input message hasno effect on the engine, but generates an output message from theprimary 602A which is read and processed by the backup 602B. The neteffect is that the backup 602B will discover any failures with theprimary in a reasonable timeframe, without having to wait for a userinput to trigger failover.

The disclosed match engine allows for order book migration—the processof copying an order book from a running primary server 602A to a newlystarted backup 602B without halting the primary. Book migration is usedwhenever a backup server 602B starts up—whether there is a working bookto migrate or not. If there is no primary server 602A up, the backup602B will wait until one exists to fully start.

When a backup 602B starts up, it gets its configuration from the FalconAdmin Server. It then sends a message to the primary server 602A tocheck if it is alive and waits for a response. There are two possiblesituations—if the primary 602A is up, it will respond right away. Ifnot, as part of the startup behavior the primary 602A sends a message onstartup. This message will be treated as a response, and the backup 602Bwill know that the primary 602A is up.

Once the primary 602A confirmation message has been received, the backup602B checks to see how much time has elapsed since it received itsconfiguration file from the FAS. If more than three seconds has elapsed,the backup reacquires the configuration from the FAS. This helpsminimize the risk that the FAS makes a change to engine configurationafter the backup 602B downloaded its configuration file, but before theprimary 602A downloaded its configuration file. Should this happen, thebackup 602B will detect this at startup and immediately fail. Thus,reacquiring the configuration file minimizes the cases of intentional“fast fail.”

Following this process the backup sends a state request message to theprimary 602A. The primary 602A responds with a state aggregate response,which contains all the mutable state—database sequence numbers,outstanding orders, host order numbers (“HON's”), host trade numbers(“HTN's”), etc. The backup 602B receives and applies this information,and is then ready to perform as a backup. In order to fully eliminatethe risk of an admin server change not being applied during thisprocess, the backup 602B compares the last sequence numbers received bythe primary from the FAS to those it received from the FAS in theconfiguration object. There is an extremely small risk of the sequencenumbers differing, but when they do it indicates that there was a userinitiated configuration change from the admin server in the few secondsbetween receipt of the configuration and receipt of the aggregate state.If this condition is detected, the backup 602B shuts down and must berestarted to attempt book migration again—it does not indicate apersistent error condition (referred to as a “fast fail” above).

Once up, the backup 602B will queue input from all sources (other orderentry gateways Admin server, internal timer events), but will notattempt to detect primary 602A failure until one failure timeout haselapsed after receipt of the first primary response message. No messagereceived before that time can cause failover.

Primary 602A startup is fairly simple. The primary 602A downloads itsconfiguration from the Falcon Admin Server, and sends a message on theshared engine bus to determine if there are any other primary enginesout there. If it receives a response, it shuts down. If not, it startsup normally and sends out a message indicating that it has started.

The market data fault tolerant policy has no role other than to suppressthe output of market data messages from the backup 602B. It does not dovalidation or checking, nor does it subscribe to the market data feedfrom the primary 602A. All important events are logged at an appropriatelog level.

The Verify-Failover-Conditions Process performs the following functions:

-   -   1) Wait ½ of the failover timeout. If appropriate RESPONSE        messages from the Primary 602A arrive during this time, then no        failover is required.    -   2) Send a “ping” message to the other order entry gateways over        the Order Entry 606 bus and listen for replies.    -   3) Wait ½ of the failover timeout. If appropriate RESPONSE        messages from the Primary 602A arrive during this time, then no        failover is required.    -   4) If a ping reply has been received over the Order Entry bus        606, then the Backup 602A can failover. If not, then the Backup        602A does not failover and instead sends an alert out to        surveillance.

The Switch-to-Primary-Mode Process performs the following functions:

-   -   1) Backup locks primary 602A out of the database. This is        achieved by entering “blocking” values in the MSG_SEQ_TABLE.        When the primary 602A attempts to write these values, it        interprets the resulting constraint violation as a backup 602 b        taking over (refer to FIG. 10).    -   2) Backup 602 b sends “Primary cease” message to the Falcon        Admin Server.    -   3) The Falcon Admin Server sends a “Primary cease” message to        the failing Primary 602A using a messaging protocol such as        Tibco, a messaging protocol promulgated by Tibco, Inc., located        in Palo Alto, Calif.    -   4) Primary 602A goes silent    -   5) Backup 602B assesses Primary's 602A state, to duplicate        whatever ordering the Primary 602A left off with. This is done        by querying the database using the last forwarded input message        as a key—the database will return the order of any subsequent        messages written by the primary 602A.    -   6) Backup 602B takes over as Primary 602A.    -   7) Backup 602B forwards the input messages in to the match        server in the same order that the primary 602A processed them.        If it has not yet received an input, it will wait for it.        Outputs resulting from these messages will be sent as possible        duplicate (“posdup”). If a message, that was not written by the        Primary 602A, is older than the message timeout parameter, it        will NOT be forwarded into the match server. Instead, it will be        discarded and a message to that effect will be logged.    -   8) Backup 602B now acts as primary 602A, sending all input        messages in to the primary 602A and sending output to both the        market data and order entry buses 606.

The following failure cases are detected:

-   -   1. The primary 602A has failed and is not sending messages. In        this case, partial or no output will be received for a        particular input message, and no output will be received for        subsequent input messages. In this case, once the backup 602B        has verified that it is still connected, and that the failure        lies with the primary 602A, it will initiate the failover        process.        -   Result: FAILOVER    -   2. A bus or network problem occurs, preventing the primary 602A        from receiving input or sending output. In this case, the        primary 602A will be unable to communicate and failover will        proceed as above.        -   Result: FAILOVER    -   3. The primary 602A has discovered a problem with its network        interfaces, database 604, or hardware. In this case, the primary        602A will go silent and failover will proceed as above.        -   Result: FAILOVER    -   4. Operations staff initiates failover from the Falcon Admin        server. In this case, failover will occur immediately, without        the need to wait a failover timeout.        -   Result: FAILOVER    -   5. Network conditions cause the primary 602A to not receive an        input message. In this case, the backup 602B will receive an        input message, but the timeout will expire without it receiving        any responses from the primary 602A relating to that message. In        a busy market, as long as the primary 602A is sending responses,        this will not initiate failover. In a quiet market, it will.        -   Result: FAILOVER or NORMAL    -   6. Network conditions cause partial loss of response messages        for a given input between the primary 602A and backup 602B. In        this case, some (but not all) output messages for a given input        message are received, and at least one response for subsequent        input is received. The backup 602B can determine the sequence of        messages processed by the primary 602A by using a combination of        the in-response-to and previous fields. The disparity in output        will be noted by the FT Message Managers.        -   Result: NORMAL    -   7. A bus or network problem occurs, preventing the backup 602B        from receiving input or primary 602A responses. In this case,        the backup 602B will attempt the ping at the failover time. The        ping will fail, and the backup 602B will not attempt to take        over.        -   Result: BACKUP HALTS. RESTARTED OR NEW BACKUP STARTED.    -   8. Network conditions cause total loss of response messages for        a given input between the primary 602A and backup 602B, assuming        there was more than one response message sent. In this case, all        output messages for a given input message are dropped between        the primary 602A and the backup 602B, and at least one response        for subsequent input is received. Even using the previous        message indicator the backup 602B cannot know how many input        messages were processed by the primary in during the gap in        output messages. As a result, it can no longer ensure that it is        processing messages in the same sequence as the primary 602A.        -   Result: BACKUP MUST BE HALTED. RESTARTED OR NEW BACKUP            STARTED.    -   9. Network conditions cause the backup 602B to not receive an        input message. In this case, the backup 602B will receive a        message in response to an input message that it has not        received.        -   Result: BACKUP MUST BE HALTED. RESTARTED OR NEW BACKUP            STARTED.    -   10. The output messages from the primary 602A and backup 602B        differ. In this case, either the primary has generated more        response messages than the backup, or the content differs. The        error will be logged and reported at high priority, and the        backup will no longer act as a backup.        -   Result: BACKUP HALTS. RESTARTED OR NEW BACKUP STARTED.    -   11. Database error, recoverable in under FAILURE_TIMEOUT. Backup        602B may prepare for failover, but primary 602A will be able to        write before backup 602B finishes taking over.        -   Result: NORMAL    -   12. Database error, not recoverable in under FAILURE_TIMEOUT.        Primary 602A will be unresponsive for the failure timeout, and        backup 602B will attempt to take over. Backup 602B will be        unable to take over immediately, but as soon as database is        recovered backup will lock primary 602A out and assume primary        602A roles.        -   Result: FAILOVER.

FIG. 9 depicts a more detailed block diagram demonstrating operation ofthe fault tolerant system of FIG. 6.

The backup 602B locks out the primary 602A, and thereby prevents a“run-away primary,” by writing “blocking” data to the message sequencetable. Because the primary 602A cannot send out a message until theinput associated with that message is persisted, the backup 602B knowsthat only messages written prior to the blocking data can be sent by theprimary 602A.

FIG. 10 depicts exemplary operation of a database 604 for use with thefault tolerant system of FIG. 6.

FIG. 11 depicts exemplary data structures utilized by the fault tolerantsystem of FIG. 6 to provide order history and trade data integrity data.

After the backup 602B which is taking over marks that it is now theprimary 602A in the database, it then asks the database for any InputMessage Identifiers that the backup may have missed the primaryprocessing (by stating the last Input ID that it saw). The backup 602Bwill then run those Inputs in the same sequence as the primary 602Aprocessed them. The database procedure will move those entries from theOrder History table to another ‘Failover Order History’ table, and fromthe Trades table to another ‘Failover Trades’ table, since the databaseknows that all orders entered after the Input ID that the backup asksabout are invalid (and will be replayed by the backup that is takingover). In this way the Order History and Trades tables are keptconsistent for inquiries, while still having a complete record of whatoccurred.

-   -   Re-play of data during Application Failover    -   Backup engine processes messages in the same order as primary.    -   In the event of failover, Backup identifies last known message.        Message is identified by MSG_SEQ, which is tagged to every        inbound record in Orders and Trades table.    -   DB picks up all Orders and Trade data written by Primary after        the last known message.    -   Move data out of ORDERS and TRADES table into failover tables.    -   No data is lost as primary data is moved to failover tables.        Useful for:        -   Verification of backup engine processing        -   Settlement of disputes        -   Fulfill regulatory needs    -   No updates required as backup (now primary) strictly does        inserts of re-play data.    -   FAS always get consistent view of the information

i. EXAMPLE

Message Sequence

Msg Msg Seq Type Server Id Machine ID 117630 FIX FAQAAPF3 FAQAAPF3117631 FIX FAQAAPF3 FAQAAPF3 117632 MATCH FAQAAPF3 FAQAAPF2 117633 MATCHFAQAAPF3 FAQAAPF2 117634 MATCH FAQAAPF3 FAQAAPF2 117762 FIX FAQAAPF3FAQAAPF2 117763 FIX FAQAAPF3 FAQAAPF2

Failover Order History

Order Event Msg Machine Order ID HON ID Seq ID Server ID Qty Status112883 112883 165532 117630 FAQAAPF3 FAQAAPF3 1 WORKING 112884 112884165533 117631 FAQAAPF3 FAQAAPF3 1 WORKING

Order History

Order Event Msg Machine Order ID HON ID Seq ID Server ID Qty Status112883 112883 165532 117762 FAQAAPF2 FAQAAPF3 1 WORKING 112884 112884165533 117763 FAQAAPF2 FAQAAPF3 1 WORKING

For Example:

-   -   Input 1, 2, 5, 3, 4, 6 are received by the Primary.    -   Input 2, 1, 5, 4, 6, 3 are received by the Backup.    -   Primary processes input 1, 2, 5, 3, 4, and writes in it's        database.    -   Backup receives primary's output for 1, 2, 5. Backup processes        these.    -   Backup then receives nothing for more than the timeout period        due to primary NIC failure.    -   Backup decides to take over.    -   Backup blocks out primary from the database.    -   Backup requests the input the Primary processed after input 5.    -   Database moves data resulting from input 3 and 4 from the Order        History table to the Order Overlap table, and from Trades to        Trades Overlap table.    -   Database returns input id's 3 and 4.    -   Backup-now-Primary processes 3 and 4 as a primary, writes data        to database    -   Backup-now-Primary sends out messages for 3 and 4 as pos-dup.    -   Backup-now-Primary now processes 6 as a primary.

It is therefore intended that the foregoing detailed description beregarded as illustrative rather than limiting, and that it be understoodthat it is the following claims, including all equivalents, that areintended to define the spirit and scope of this invention.

1. A method of providing fault tolerance to a primary instance, themethod comprising: receiving a copy of a first input transmitted to theprimary instance; forwarding the copy of the first input to a backupinstance operative to generate a first backup result based on theforwarded copy of the first input; waiting, in response to the receivingof the copy of the first input, for the primary instance to transmit afirst primary result based on the first input; determining that thefirst primary result is not likely to be transmitted; preventing, basedon the determination that the first primary result is not likely to betransmitted, the primary instance from completing a transaction that theprimary instance is supposed to complete to continue operating; andtransmitting the first backup result when the first primary result isnot likely to be transmitted.
 2. The method of claim 1 furthercomprising: receiving a copy of a second input transmitted to theprimary instance, the copy of the second input being received subsequentto the copy of the first input; waiting, in response to the receiving ofthe copy of the second input, for the primary instance to transmit asecond primary result based on the second input; wherein the forwardingfurther comprises forwarding the copy of the first input to the backupinstance upon the transmission of the first and second primary resultsby the primary instance; and comparing the first primary result with thefirst backup result and indicating a failure of the backup instance, theprimary instance, or a combination thereof, when the first primaryresult is at least partially different from the first backup result. 3.The method of claim 1, wherein the waiting further comprises waiting fora defined period of time to elapse from when the first input isreceived, wherein the determining further comprises determining that thefirst primary result has not been received before the defined period oftime has elapsed.
 4. The method of claim 1, wherein the transactioncomprises a store transaction to a database, the preventing furthercomprising preventing completion of the store transaction.
 5. The methodof claim 4, wherein the preventing further comprises causing thedatabase to return a constraint violation in response to the storetransaction.
 6. The method of claim 1, where in the primary instancecomprises a match server of a financial exchange.
 7. The method of claim1, wherein the primary instance comprises a software application.
 8. Themethod of claim 1, wherein the primary instance comprises a processor.9. A system for providing fault tolerance to a primary instance, thesystem comprising: a receiver operative to receive a copy of a firstinput transmitted to the primary instance; an input forwarder coupledwith the receiver and operative to forward the copy of the first inputto a backup instance, the backup instance being operative to generate afirst backup result based on the forwarded copy of the first input; afault detector coupled with the receiver and the primary instance andoperative to wait, in response to the receiving of the copy of the firstinput, for the primary instance to transmit a first primary result basedon the first input, wherein the fault detector is further operative todetermine that the first primary result is not likely to be transmitted;a transaction inhibitor coupled with the fault detector and operative toprevent, based on the determination that the first primary result is notlikely to be transmitted, the primary instance from completing atransaction that the primary instance is supposed to complete tocontinue operating; and a transmitter coupled with the backup instanceand the fault detector and operative to transmit the first backup resultwhen the first primary result is not likely to be transmitted.
 10. Thesystem of claim 9 wherein: the receiver is further operative to receivea copy of a second input transmitted to the primary instance, the copyof the second input being received subsequent to the copy of the firstinput; the fault detector is further operative to wait, in response tothe receiving of the copy of the second input, for the primary instanceto transmit a second primary result based on the second input; whereinthe input forwarder is further operative to forward the copy of thefirst input to the backup instance upon the transmission of the firstand second primary results by the primary instance; the system furthercomprising: a comparator coupled with the primary instance and thebackup instance and operative to compare the first primary result withthe first backup result and indicate a failure of the backup instance,the primary instance or a combination thereof, when the first primaryresult is at least partially different from the first backup result. 11.The system of claim 9, wherein the fault detector is further operativeto wait for a defined period of time to elapse from when the first inputis received and determine that the first primary result has not beenreceived before the defined period of time has elapsed.
 12. The systemof claim 9, wherein the transaction comprises a store transaction to adatabase, the transaction inhibitor being further operative to preventcompletion of the store transaction.
 13. The system of claim 12, whereinthe transaction inhibitor being further operative to cause the databaseto return a constraint violation in response to the store transaction.14. The system of claim 9, where in the primary instance comprises amatch server of a financial exchange.
 15. The system of claim 9, whereinthe primary instance comprises a software application.
 16. The system ofclaim 9, wherein the primary instance comprises a processor.
 17. Asystem for providing fault tolerance to a primary instance, the systemcomprising: a processor; a memory coupled with the processor; firstlogic stored in the memory and executable by the processor to receive acopy of a first input transmitted to the primary instance; second logicstored in the memory, coupled with the first logic and executable by theprocessor to forward the copy of the first input to a backup instance,the backup instance being operative to generate a first backup resultbased on the forwarded copy of the first input; fourth logic stored inthe memory, coupled with the first logic and executable by the processorto wait, in response to the receiving of the copy of the first input,for the primary instance to transmit a first primary result based on thefirst input, wherein the fourth logic is further executable by theprocessor to determine that the first primary result is not likely to betransmitted; fifth logic stored in the memory, coupled with the fourthlogic and executable by the processor to prevent, based on the fourthlogic determining that the first primary result is not likely to betransmitted, the primary instance from completing a transaction that theprimary instance is supposed to complete to continue operating; andsixth logic stored in the memory, coupled with the backup instance andthe fourth logic and executable by the processor to transmit the firstbackup result when the first primary result is not likely to betransmitted.
 18. The system of claim 17 wherein: the first logic isfurther executable by the processor to receive a copy of a second inputtransmitted to the primary instance, the copy of the second input beingreceived subsequent to the copy of the first input; the fourth logic isfurther executable by the processor to wait, in response to thereceiving of the copy of the second input, for the primary instance totransmit a second primary result based on the second input; wherein thesecond logic is further executable by the processor to forward the copyof the first input to the backup instance upon the transmission of thefirst and second primary results by the primary instance; the systemfurther comprising: seventh logic stored in the memory, coupled with theprimary instance and the backup instance and executable by the processorto compare the first primary result with the first backup result andindicate a failure of the backup instance, the primary instance or acombination thereof, when the first primary result is at least partiallydifferent from the first backup result.
 19. The system of claim 17,wherein the primary instance comprises a first execution by theprocessor of seventh logic stored in the memory and executable by theprocessor, the backup instance comprising a second execution of theseventh logic by the processor.
 20. A system for providing faulttolerance to a primary instance means, the system comprising: means forreceiving a copy of a first input transmitted to the primary instance;means for forwarding, coupled with the means for receiving, the copy ofthe first input to a backup instance means operative to generate a firstbackup result based on the forwarded copy of the first input; means forwaiting, in response to the receiving of the copy of the first input,for the primary instance to transmit a first primary result based on thefirst input, the means for waiting being coupled with the means forreceiving; means for determining, coupled with the means for receivingand the means for waiting, that the first primary result is not likelyto be transmitted; means for preventing, coupled with the means fordetermining, the primary instance means from completing a transactionthat the primary instance means is supposed to complete to continueoperating, based on the determination that the first primary result isnot likely to be transmitted; and means for transmitting, coupled withthe means for determining and the backup instance means, fortransmitting the first backup result when the first primary result isnot likely to be transmitted.
 21. The system of claim 20 furthercomprising: means for receiving a copy of a second input transmitted tothe primary instance means, the copy of the second input being receivedsubsequent to the copy of the first input; means for waiting, inresponse to the receiving of the copy of the second input, for theprimary instance means to transmit a second primary result based on thesecond input; and wherein the means for forwarding further comprisesmeans for forwarding the copy of the first input to the backup instancemeans upon the transmission of the first and second primary results bythe primary instance; the system further comprising: means for comparingthe first primary result with the first backup result and indicating afailure of the backup instance, the primary instance or a combinationthereof, when the first primary result is at least partially differentfrom the first backup result, the means for comparing being coupled withthe primary instance means and the backup instance means.